Regulatory agencies throughout the world have established standards and regulations for connecting subscriber equipment to telephone networks. These regulations are intended to prevent damage to the telephone network and mitigate interference with other equipment also connected to the network. The regulations, however, often present difficult design challenges.
For example, subscriber equipment or data communications equipment, such as a data modem, is generally required to provide for some form of electrical isolation to prevent voltage surges or transients originating from the subscriber equipment from having a deleterious effect on the telephone network. Electrical isolation also addresses potential problems associated with differences in operating voltages between a telephone line and the subscriber equipment. More particularly, telephone line voltages may vary widely across a given network, and often exceed the operating voltage of subscriber equipment. In the United States, 1,500-volt isolation is currently required. In other countries, the prescribed isolation may reach 3,000-4,000 volts.
A number of techniques have been utilized to provide the requisite level of electrical isolation. For example, large analog isolation transformers are often employed to magnetically couple analog signals between a two-wire telephone line and the analog front end of a modem or other circuit while maintaining an appropriate level of electrical isolation. The isolation transformer functions to block potentially harmful DC components, thereby protecting both sides of the data connection.
The isolation transformer is typically part of what is referred to in the modem arts as a data access arrangement (DAA). The term DAA generally refers to circuitry that provides an interface between a public telephone network originating in a central office and a digital data bus of a host system or data terminal equipment. The DAA electrically isolates a modem or similar device from a phone line to control emissions of electromagnetic interference/radio frequency interference (EMI/RFI). In addition to electrical isolation, the DAA often develops a number of signals (e.g., a ring signal) for provision to subscriber equipment. The DAA may receive signals from the phone line through a telephone jack, such as a RJ11C connection as used for standard telephones.
Typically, a number of circuits must derive information from the telephone line, and isolation is often required for each signal communicated to and from the host system. Such circuits may include: transmit and receive circuitry; ring signal detection circuitry; circuitry for switching between voice and data transmissions; circuits for dialing telephone numbers; line current detection circuitry; circuitry for indicating that the equipment is coupled to a functional telephone line; and line disconnection detection circuitry. Conventional DAA designs utilize separate line side circuits and separate signal paths across a high voltage isolation barrier for each function of the DAA. This conventional design requires an undesirably large number of isolation barriers.
A more modern solution to reduce the number of isolation barriers in a DAA is to separate the DAA circuitry into line-side circuitry and system side circuitry. The line-side circuitry includes the analog components required to connect to the telephone line, while the system side circuitry typically includes digital signal processing circuitry and interface circuitry for communicating with the host system. Incoming analog data signal from the telephone line is digitized via an analog-to-digital converter in the line-side circuitry and transmitted across the “digital” isolation barrier to the system side circuitry via a digital bi-directional serial communication link. The digital data signal may then be processed by the digital signal processing circuitry in the system side circuitry. Conversely, digital data signals from the host system may be transmitted via the bi-directional serial communication link through the digital isolation barrier to the line side circuitry, where the digital data signals are converted to analog signals and placed on the telephone line.
A problem that arises in this more modern DAA, however, is that the line-side circuitry must be provided with a separate DC power supply that is isolated from the host system power. Two main approaches to provide an isolated power supply have been proposed. In the first approach, power is transferred from the host system to the line side circuitry via a separate power transformer in the form of a stream of digital pulses. The pulses form an AC signal that may be converted to a DC supply voltage via a rectifier in the line-side circuitry. This approach disadvantageously requires at least two transformers—one to serve as the isolation barrier for the digital data signals, and the other to provide power to the line-side circuitry.
A second approach that has been proposed is to derive power for the line-side circuitry from the telephone line itself. This approach, however, is difficult to implement in practice, because the specifications of the telephone communications systems in certain countries, including Germany and Austria, severely limit the amount of power that a DAA may use from a telephone line. This approach also tends to reduce the distance that the subscriber equipment may be located from the telephone central office, because the voltage drop on the telephone line increases as the distance between the subscriber equipment and the telephone company central office increases.